Polymer particles can be prepared by a process frequently referred to as “limited coalescence”. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed, and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water-insoluble solid particulate suspension stabilizer, typically silica or latex, and the size of the solvent-polymer droplets produced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, and the like.
Limited coalescence techniques, which typically result in the formation of polymer particles having a substantially uniform size distribution, have been described in numerous patents pertaining to the preparation of electrostatic toner particles. Such procedures employed in toner preparation are described in, for example, U.S. Pat. Nos. 4,833,060 and 4,965,131, the disclosures of which are incorporated herein by reference. The described procedures typically consist of the following steps: mixing a polymer material, a solvent and, optionally, a colorant and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent; and washing and drying the resultant product.
The limited coalescence technique just described requires that the colorant be dispersed in the organic phase. However many pigments in powder form have limited solubility in water-immiscible organic solvents. Also, in many cases, colorants are received as water-wet pigment cakes. U.S. Pat. No. 5,262,268 discloses a method of forming colored toner from a pigment wet cake by melt blending it with a toner resin in an extruder, and removing water from the extruder. Colorants in wet cake form may be cheaper in price than in dry powder, flush or master batch form, but they cannot be dispersed in an organic phase by the prior art procedures described in U.S. Pat. Nos. 4,833,060 and 4,965,131. Also, when the colorant needs to be milled for improved dispersion, milling can be carried out more safely in an aqueous medium than in an organic medium, thereby providing an aqueous dispersion of the colorant suitable for use in the process of the present invention.